The present invention relates, in general, to a driver circuit for a laser, for example a laser diode. The invention is particularly, but not exclusively, concerned with matching a load impedance (that of the laser diode) to a source impedance (that of the laser driver) for maximum power transfer through the system.
It is well known that when a laser driver circuit is used to drive a laser diode many problems can occur due to mismatches between the source and the load as a result of the large difference in impedance between the two. The large mismatch in impedance between the source and the load arises because state-of-the-art laser driver circuits use an open collector circuit configuration, in which a collector of a transistor used in the driver circuit is coupled, directly or indirectly via, for example, an impedance, to the laser diode. Typically, for example, a collector of an NPN transistor would have an output impedance in the order of 1 kxcexa9 in parallel with 500 fF, whereas the laser diode typically has an input impedance of only a few ohms (for example 5 xcexa9 in parallel with 2 pF). Such a mismatch causes a proportion of the power to be reflected back from the laser diode, instead of being transmitted by the laser diode as laser light.
The present invention therefore seeks to provide a new and improved laser driver circuit having a relatively high efficiency.
Another object of the invention is to provide a new and improved laser driver circuit having a transistor driver for a laser, wherein a substantial impedance match is provided between a transistor driver and a laser diode load.
Accordingly, in a first aspect, the invention provides a laser driver circuit comprising a transistor having emitter, collector and base electrodes, the base electrode being coupled to an input terminal for receiving an input signal, the collector electrode being coupled to a first power supply rail and the emitter electrode being coupled to an output terminal for coupling to a laser.
In a preferred embodiment, the laser driver circuit comprises two transistors, each having emitter, collector and base electrodes, the respective base electrodes being coupled to respective differential input terminals for receiving a differential input signal, the respective collector electrodes being coupled to a first power supply rail and the respective emitter electrodes being coupled to a pair of respective differential output terminals for coupling to a pair of respective input terminals of a laser.
Preferably, each emitter electrode is coupled to the respective output terminal via a respective impedance. Each emitter electrode is preferably coupled to a respective current source, each current source preferably being controllable. Each current source is preferably coupled to a second power supply rail. One of the impedances is preferably coupled to either the first or the second power supply rail.
In one preferred embodiment, each emitter electrode is coupled to the respective output terminal via a respective capacitance coupled in series with the respective impedance. Preferably, each capacitance is coupled to the respective first or second power supply rail via an inductance. One of the inductances is preferably coupled to its respective power supply rail via a current source, which is preferably controllable.
In a second aspect, the invention provides a method of driving a laser using the laser driver circuit described above.